PROCESS AND APPARATUS FOR DIRECT CRYSTALLIZATION OF POLYCONDESATES

Abstract

A process for continuous production of partly crystalline polycondensate pellet material which comprises the step of crystallizing the pellet material in a second treatment space (6a) under fixed bed conditions by supply of energy from the exterior by means of a process gas, wherein the process gas has a temperature (T.sub.Gas), which is higher than the sum of the pellet temperature (T.sub.GR) and the temperature increase (T.sub.KR) which occurs due to heat of crystallization released hi the second treatment space (6a), i.e., (T.sub.Gas>(T.sub.GR+T.sub.KR)). The pellets at the exit from the second treatment space (6a) have an average temperature (T.sub.PH), which is 10 to 90 C. higher than the sum of the temperature of the pellets (T.sub.GR) and the temperature increase (T.sub.KR) which occurs due to heat of crystallization released in the second treatment space (6a), i.e., (T.sub.GR+T.sub.KR+90 C.)T.sub.PH(T.sub.GR+T.sub.KR+10).

Claims

1-15. (canceled)

16. A process for continuous production of partly crystalline polycondensate pellet material, comprising the steps of: a) forming a melt of a polycondensate into pellets by adding a liquid cooling medium, and cooling to an average pellet temperature within the range of temperature of crystallization of the polycondensate, wherein cooling takes place before or during or after forming to pellets; b) separating the liquid cooling medium from the pellets in a first treatment space, wherein the pellets after exit from the first treatment space exhibit a pellet temperature (T.sub.GR), c) crystallizing the pellets in a second treatment space, wherein in the second treatment space fixed bed conditions exist and the pellets are heated in the second treatment space by supply of energy from the exterior by a process gas, wherein the process gas has a gas temperature (T.sub.Gas), which is higher than a sum of a pellet temperature (T.sub.GR) and a temperature increase (T.sub.KR) which occurs due to heat of crystallization released in the second treatment space, (i.e., T.sub.Gas>(T.sub.GR+T.sub.KR)), and wherein the pellets at the exit from the second treatment space have an average temperature (T.sub.PH), which is 10 to 90 C. higher than the sum of the temperature of the pellets (T.sub.GR) and the temperature increase (T.sub.KR) which occurs due to heat of crystallization released in the second treatment space (i.e., (T.sub.GR+T.sub.KR+90 C.)T.sub.PH(T.sub.GR+T.sub.KR+10 C.)).

17. The process according to claim 16, wherein the polycondensate is selected from the group consisting of polyesters and polyamides.

18. The process according to claim 17, wherein the polycondensate is a polyethylene terephthalate hornopolymer or copolymer.

19. process according to claim 18, wherein drying of the pellets occurs in the first treatment space, and, after exit from the first treatment space, the pellets have a pellet temperature (T.sub.GR) in the range from 100 to 180 C.

20. The process according to claim 16, wherein the liquid cooling medium has a temperature in the range of 50 C. to 80 C.

21. The process according to claim 16, wherein a ratio (X) of a mass flows of gas (m.sub.G) and pellets (m.sub.P) (X 32 m.sub.G/m.sub.P) in the second treatment space is set such that 4 C.(T.sub.GasT.sub.KRT.sub.GR)*X400 C.

22. The process according to claim 16, wherein a process gas flows through the second treatment space in counter-current or alternatively in cross-current or a combination of counter-current and cross-current.

23. The process according to claim 16, wherein a treatment in the second treatment space takes place in several stages, and each stage comprises all feed devices emanating from a distribution space and all discharge devices emptying into an associated collection space.

24. The process according to claim 16, wherein in the second treatment space the pellets are heated to a temperature intended for a subsequent SSP reaction.

25. The process according to claim 16, wherein in step a) the liquid coding medium has a temperature below the glass transition point (Tg) of the polycondensate.

26. The process according to claim 16, wherein the pellets are heated at an outlet from the second treatment space to the average temperature (T.sub.PH) ((i.e., (T.sub.GR+T.sub.KR+90 C.)T.sub.PH(T.sub.GR+T.sub.KR+15 C.)).

27. A device comprising: a unit for forming pellets with a line for supply of a coding medium and a line for discharge of a mixture of pellets/cooling medium, a separating device arranged downstream of the unit for forming pellets and providing a first treatment space, and a unit which is arranged downstream of the separating device without a crystallization unit disposed there between and which provides a second treatment space and is equipped with an inlet opening and an outlet opening for the pellets, respectively, wherein the second treatment space is connected to at least two feed devices and at least two discharge devices for process gas so that, in the second treatment space, the process gas can be led through the pellets under conditions of a fixed bed.

28. The device according to claim 27, wherein the at least two feed devices and at least two discharge devices are arranged in such a way that the pellets, after passing through a region of the second treatment space in which the pellets are exposed to a gas flow from the at least one feed device, reach a region of the second treatment space in which the pellets are exposed to a gas flow from the at least one additional feed device.

29. The device according to cairn 27, wherein the unit with the second treatment space is a pre heater.

30. The device according to claim 28, wherein the unit with the second treatment space is a preheater.

31. The device according to claim 27, wherein the second treatment space is arranged below the first treatment space.

Description

[0140] The present invention will now be further be described in detail by reference to non-limiting drawings, where

[0141] FIG. 1 shows a schematic illustration of a preferred embodiment of an apparatus according to the present invention.

[0142] FIG. 2 shows a schematic illustration of a preferred embodiment of a downstream unit with a second treatment space in accordance with the present invention, wherein the process gas is conducted essentially in countercurrent flow.

[0143] The apparatus according to FIG. 1 has a reactor 1 for producing a polymer melt. This can be a reactor in which a melt polymerization is carried out to thus produce a prepolymer from the monomers. Alternatively, reactor 1 can also be an apparatus for melting a solid product, for example a prepolymer. Reactor 1 can in this case be an extruder, for example.

[0144] The molten material is transferred into a pelletization apparatus 2. In the pelletization apparatus 2, a pellet material is produced from the molten material in a known manner. This may be, for example, an underwater pelletizer (as shown in FIG. 1). Pelletization takes place under water in this case. The pellets obtained are concurrently cooled down in pelletizer 2. As noted above, however, cooling must not be so severe as to cool the pellets to below their crystallization temperature range. This can be achieved by the use of heated water having a temperature above 50 C., but at least 10 C. below it pressure-dependent boiling point, in particular having a temperature below the Tg of the polycondensate, especially having a temperature from 50 to 80 C. The pellet material should, in the case of polyethylene terephthalate (PET), be cooled to a temperature in the range from 110 to 180 C., preferably 115 to 160 C. and especially preferred 120 to 150 C.

[0145] The pellet material is transferred via a connection line 3 and through an inlet opening 3a into the unit for drying the pellet material (separating unit) 4. To prevent the pellet material from cooling down too much, this pellet material should be conveyed as quickly as possible out of the pelletization apparatus 2 and through the connection line 3. Preferably, the flow velocity in connection line 3 can be increased by feeding a gas stream (preferably air) into it.

[0146] The pellet material is separated from the liquid cooling medium (water) and dried in a first treatment space 4a in the unit for drying the pellet material (separating unit) 4. The cooling medium separated off is conducted through an exit opening 3c via a pipework line 9a back into the stock reservoir vessel (tank) 9b for the cooling medium. The stock reservoir vessel 9b has an inlet 9e for importation of cooling medium. From the stock reservoir vessel 9b, the cooling medium is transferred into the pelletization apparatus 2 by means of a circulation apparatus (pump) 9c. In the course of the transfer, the cooling medium preferably traverses a heat exchanger 9d. In the heat exchanger 9d, the cooling medium can be heated or cooled down, as required. Especially cooling medium returned from the separating unit 4 can have an excessive temperature because of the contact with hot pellet material and has to be cooled before entry into the pelletization apparatus 2.

[0147] The fresh cooling medium, which is added via the inlet 9e, can contain a basic medium or a pH buffer medium. In particular, the use of water with a neutralizing or buffer effect which is adjusted within a narrow range is provided herein. Alternatively, the addition of a basic medium or of a pH buffer medium can also be effected directly into the cooling circular system, e.g. into the storage container 9b.

[0148] The drying of the pellet material in the first treatment space of unit 4 is effected at a temperature of 100 to 200 C., preferably 120 to 160 C., by means of air, or a gas atmosphere comprising essentially air, besides a mechanical separation unit. In the apparatus of FIG. 1, the air is fed into the separating unit 4 via an inlet opening 10a. The inlet opening 10a for the gas can be situated in the housing of separating unit 4 or in the connection line 5 or in both locations. Optionally, an aspirating filter (not shown) can be disposed in the gas line leading to the inlet opening 10a. The air leaves the separating unit 4 through the outlet opening 10b. A ventilator 10c for circulating the air through the separating unit 4 is provided in the gas line 10c leading away from the exit opening 10b in the apparatus of FIG. 1. However, the ventilator could alternatively also be provided in the air inlet line 10a. Furthermore, inlet opening 10a and outlet opening 10b can be connected to each other to form a circuit system. A condenser would then have to be provided in this circuit system.

[0149] The pellets are transferred from the separating device 4 through a discharge opening 3b via a connecting line 5 via a cellular wheel lock 11 through an inlet opening 6b into a unit 6 with a second treatment space 6a. The pellets can pass unhindered from the separating device 4 into the unit 6. In this embodiment, unit 6 is a preheater.

[0150] In unit 6, the essentially amorphous pellet material is at least partially crystallized in a second treatment space 6a. Within the second treatment space 6a, the pellets are thermally treated by a gas stream passing through unit 6 in countercurrent or crosscurrent. Within the second treatment space 6a, the conditions are those of a fixed bed.

[0151] The pellets are crystallized by external heat supply, wherein for external heat supply a process gas in counter current to the pellet flow is led through the second treatment space 6a, said process gas having a temperature T.sub.Gas which is higher than the sum of the pellet temperature T.sub.GR and the temperature increase T.sub.KR in the second treatment space 6a which occurs due to released heat of crystallization, i.e. T.sub.Gas>(T.sub.GR+T.sub.KR). In the case of polyethylene terephthalate (PET) crystallization occurs due to heating to a temperature of 170 to 235 C., wherein a process gas having a temperature T.sub.Gas of 175 bis 240 C. is used. At temperatures above 180 C. there is preferably used an inert gas, essentially nitrogen, as process gas. The crystallized pellet material exits the unit 6 through an exit opening 6c via a discharge apparatus 7, for example a barrier apparatus such as a cellular wheel lock.

[0152] Alternatively, downstream of the cellular wheel lock there can be provided a second wheel unit (such as a cellular wheel lock).

[0153] The pellets can be subjected to a subsequent thermal treatment such as a dealdehydization or SSP reaction. Alternatively, the pellets can also be sent into a cooling step.

[0154] The process gas used in unit 6 is conducted through a closed-loop circuit system of pipework lines 8a. The process gas enters into unit 6 through an inlet opening 6d and exits the opening 6 through an outlet opening 6e. The circuit system for the process gas contains a ventilator 8b for circulating the gas. A heat exchanger 8c is provided upstream of the inlet opening 6d to bring the gas to the desired temperature before entry into the unit 6. Preferably, the gas is heated in heat exchanger 8c.

[0155] FIG. 2 shows a schematic representation of a preferred embodiment of a downstream unit 6 in the form of a countercurrent preheater with a second treatment space 6a according to the present invention, as described for example in Scheirs/Long (ed.), Modern Polyesters, Wiley 2003, p. 170.

[0156] This preferred embodiment of a downstream unit 6 with a second treatment space 6a also has an inlet opening 6b and an outlet opening 6c for the polycondensate pellets, as described above for FIG. 1.

[0157] This particular embodiment is characterized by the fact that the inlet opening 6d is located in a feed device 6f. In front of the feed device 6f there is a distribution space 6g through which process gas is fed to the feed device 6f. Several feeding devices 6f with corresponding inlet openings 6d can be provided.

[0158] At least one feed opening 6h for process gas empties into this distribution space 6g.

[0159] This particular embodiment is further characterized by the fact that the outlet opening 6e is located in a discharge device 6i. There may be several discharge devices 6i with corresponding outlet openings 6e. Behind the discharge devices 6i there is a collection space 6j in which process gas from the discharge devices 6i is collected. At least one discharge opening 6k for process gas empties into this collection space 6j.

[0160] Furthermore, devices for the distribution of the process gas, such as baffles, valves or flaps, as well as separate channels for individual process gas supply and removal can be arranged. Devices can be located in or below the discharge device 6i which allow the passage of process gas but obstruct the passage of pellets. This can be done, for example, by means of a bent or deflected flow channel or with the aid of deflecting installations such as a zigzag separator.

[0161] Preferably a plurality of roof-shaped or tubular feed devices 6f, into which process gas coming from a distribution space 6g flows, are located on one plane, and above this plane on a further plane a plurality of roof-shaped or tubular discharge devices 6i, through which process gas flows into a collection space 6j, are located, wherein the process gas flows from the lower plane to the upper plane and in so doing flows through the bulk material, which flows through the second treatment space 6a from top to bottom, in countercurrent.

[0162] The feeding devices 6f and the discharge devices 61 are arranged in such a way that a direct gas flow from the distribution space 6g to the collection space 6j is avoided.

[0163] Furthermore, the preferred embodiment according to FIG. 2 is characterised in that below and in addition to at least one feed device 6f and discharge device 6i for process gas, at least one further feed device 6f2 and discharge device 6i2 for process gas opens into the second treatment space 6a. The inlet opening 6d2 is located in the feed device 6f2. The feed device 6f2 also opens into the distribution space 6g through which process gas is fed to the feed device 6f2.

[0164] The inlet opening 6e2 is located in the discharge device 6i2. The discharge device 6i2 also empties into the collection room 6j.

[0165] The at least one further feed device 6f2 and discharge device 6i2 are arranged such that the polycondensate pellets are exposed to the gas flow from the at least one additional feed device 6f2 after passing through a region of the second treatment space 6a in which it has been exposed to a gas flow from the at least one feed device 6f. This allows a multi-stage heat supply to be achieved.

[0166] FIG. 3 shows a schematic representation of another preferred embodiment of a downstream unit 6 with a second treatment space 6a in accordance with the present invention.

[0167] The second treatment space 6a is essentially a vertically aligned slot in a round, cylindrical jacket of the downstream unit 6.

[0168] This preferred embodiment of a downstream unit 6 with a second treatment space 6a also has an inlet opening 6b and an outlet opening 6c for the polycondensate pellets, as described above in FIGS. 1 and 2.

[0169] This preferred embodiment of a downstream unit 6 with a second treatment space 6a also features a feed device 6f and a discharge device 6i for the process gas.

[0170] The particular embodiment according to FIG. 3 is characterised by the fact that the inlet opening 6d is located in a feed device 6f. The feed device 6f is preferably equipped with a large number of inlet openings 6d, wherein the inlet openings 6d are selected in such a way that process gas, but not pellets, can flow through them. A preferred embodiment is that the feed device 6f comprises a perforated plate or a slotted screen in which the inlet openings 6d are arranged.

[0171] At least one 6h feed opening for process gas opens into the 6f feed device.

[0172] This particular embodiment is further characterised by the fact that the outlet opening 6e is located in a discharge device 6i. Preferably, the discharge device 6i is provided with a multitude of outlet openings 6e, wherein the outlet openings 6e are selected in such a way that process gas, but not pellets, can flow through them. A preferred embodiment is that the discharge device 6i comprises a perforated plate or a slotted screen in which the outlet openings 6e are arranged.

[0173] At least one discharge opening 6k for process gas flows into the discharge device 6i.

[0174] Furthermore, devices for the distribution of the process gas, such as baffles, valves or flaps, as well as separate channels for individual process gas supply and removal, can be provided.

[0175] The feed devices 6f and the discharge devices 6i are preferably located essentially on the same plane, wherein the process gas flows from the feed device 6f to the discharge device 6i and in so doing flows through the bulk material, which flows through the second treatment space 6a from top to bottom, in a cross-current.

[0176] Furthermore, the preferred embodiment according to FIG. 3 is characterised in that below and in addition to at least one feed device 6f and discharge device 6i for process gas, at least one further feed device 6f2 and discharge device 6i2 for process gas opens into the second treatment space 6a. The at least one further feed device 6f2 and discharge device 6i2 are arranged such that the polycondensate pellets are exposed to the gas flow from the at least one additional feed device 6f2 after passing through a region of the second treatment space 6a in which it has been exposed to a gas flow from the at least one feed device 6f. This allows a multi-stage heat supply to be achieved.

[0177] The present apparatus of the present invention is very useful for continuous pelletization and crystallization of a polymer, especially a polycondensate, preferably a polyester such as polyethylene terephthalate.

[0178] The relationship between gas temperature and gas to product ratio to product temperature can be illustrated by the following non-restrictive examples:

EXAMPLE 1

[0179] In a round, cylindrical treatment space with a diameter of 1.9 m and a vertical distance of 1.2 m between the gas inlet and outlet, 10 t/h of PET pellets with an initial crystallinity of 2.5% and an inlet temperature of T.sub.GR=140 C. were treated.

[0180] When 3000 kg/h (X=0.3) of nitrogen with T.sub.Gas=220 C. were added in counter-current, heating to T.sub.PH=171 C. and an increase in crystallinity to 32.5% took place, which corresponds to a temperature increase due to crystallization T.sub.KR=18.7 C. For the heat of crystallization q=115 J/g is used for 100% crystallinity, and for the heat capacity c=1.84 J/g/K is used.

[0181] T.sub.Gas is therefore 61.3 C. higher than T.sub.GR+T.sub.KR, and X*(T.sub.GasT.sub.GRT.sub.KR) is 18.4 C., and T.sub.PH is 12.3 C. higher than T.sub.GR+T.sub.KR.

[0182] The gas velocity at the gas outlet was 0.36 m/s, which meant that fixed bed conditions were present. The result was a pressure drop of 34 mbar. The residence time of the pellets in the treatment space was 16.3 minutes.

EXAMPLE 2

[0183] When the amount of gas in example 1 was increased to 6000 kg/h (X=0.6), heating to T.sub.PH=181.2 C. took place. The increase in crystallinity and thus T.sub.KR remain unchanged.

[0184] T.sub.Gas remains 61.3 C. higher than T.sub.GR+T.sub.KR, and X*(T.sub.GasT.sub.GRT.sub.KR) is 36.8 C., and T.sub.PH is 24.2 C. higher than T.sub.GR+T.sub.KR.

[0185] The gas velocity at the gas outlet was 0.73 m/s, which meant that fixed bed conditions were still present. There was a pressure drop of 66 mbar.

[0186] This example shows the better heating with a higher amount of gas in the fixed bed.

Comparative Example 1

[0187] When the amount of gas in example 1 was increased to 8000 kg/h (X=0.8), the gas velocity at the gas outlet was >1 m/s, resulting in local fluidized bed conditions. Only heating to T.sub.PH=178.8 C. took place.

[0188] The result was a pressure drop of 75 mbar.

[0189] This example shows the worse heating in spite of higher amount of gas, as well as the higher pressure drop, when the heating is no longer carried out under fixed bed conditions.

EXAMPLE 3

[0190] When the treatment space in example 1 was divided into 4 zones of 0.3 m height and the product was treated in each zone with an amount of gas of 6000 kg/h (X total=2.4), heating to T.sub.PH=206 C. was carried out.

[0191] With T.sub.KR unchanged, X*(T.sub.GasT.sub.GRT.sub.KR) is 166 C., and T.sub.PH is 47.3 C. higher than T.sub.GR+T.sub.KR.

[0192] The average gas velocity at the gas outlet was 0.79 m/s, which meant that fixed bed conditions were still present. This resulted in a pressure drop of 71 mbar.

[0193] This example shows the better heating with a multi-stage process.

Comparative Example 2

[0194] When the second treatment space was provided in a simple crystallization vessel, at least an H/D ratio of 2 had to be maintained for sufficient product and gas distribution. With a residence time of 7.5 minutes and a throughput of 10 t/h, a crystallization vessel with a diameter of 1 m and a height of 2 m was to be used. The PET pellets with an initial crystallinity of 2.5% were treated with an inlet temperature of T.sub.GR=140 C.

[0195] When 1800 kg/h (X=0.18) of nitrogen with T.sub.Gas=220 C. were added in counter-current, heating to T.sub.PH=162 C. and an increase in crystallinity to 25.3% took place, which corresponds to a temperature increase due to crystallization T.sub.KR=14.2 C.

[0196] T.sub.Gas is therefore 65.8 C. higher than T.sub.GR+T.sub.KR, and X*(T.sub.GasT.sub.GRT.sub.KR) is 11.8 C., and T.sub.PH is 7.8 C. higher than T.sub.GR+T.sub.KR.

[0197] The gas velocity at the gas outlet was 0.78 m/s, which meant that fixed bed conditions were still present. However, there was a pressure drop of 112 mbar.

[0198] This example shows the poorer heating in a conventional crystallization vessel with a simultaneous high pressure drop. A significantly higher amount of gas cannot be used, as otherwise fixed bed conditions no longer exist.

Comparative Example 3

[0199] When comparative example 2 was carried out with a product throughput of 50 t/h, a crystallization vessel with a diameter of 1.75 m and a height of 3.5 m was to be used with a residence time of 8 minutes.

[0200] When 5500 kg/h (X=0.11) of nitrogen with T.sub.Gas=220 C. were added in counter-current, heating to T.sub.PH=159.4 C. and an increase in crystallinity to 25.9% took place, which corresponds to an increase in temperature due to crystallization T.sub.KR=14.6 C.

[0201] T.sub.Gas is therefore 65.4 C. higher than T.sub.GR+T.sub.KR, and X*(T.sub.GasT.sub.GRT.sub.KR) is 7.2 C., and T.sub.PH is 4.8 C. higher than T.sub.GR+T.sub.KR.

[0202] The gas velocity at the gas outlet was 0.77m/s, which meant that fixed bed conditions were still present. However, there was a pressure drop of 188 mbar.

[0203] This example shows how the situation deteriorates with increasing throughput with a conventional crystallization vessel.